Luciferin is a substrate of a bioluminescence enzyme, luciferase, and after emitting light as a result of luciferase reaction, is converted to oxyluciferin. ATP measurement methods using luciferase are widely used in the fields of medical science and food hygiene. However, luciferin which is used as a substrate, is expensive and the luciferase reaction is inhibited by oxyluciferin produced after reaction. Thus, removal of oxyluciferin or regeneration to luciferin will enable further development of the ATP measurement methods using luciferase. A protein which is derived from a firefly and capable of regenerating luciferin from oxyluciferin has been found (U.S. Pat. No. 5,814,504). However, only a small quantity of the protein can be extracted from a firefly body so that industrial application of the protein has been difficult.
Addition of such a protein capable of regenerating luciferin to the luciferin-luciferase reaction system enables improvement in durability of luminescence and reduction in the amount of luciferase and luciferin to be used.